1. Field of the Invention
The present invention relates to a method to concentrate and detect radioactive gases. In particular it relates to a method to concentrate and detect radioactive gases released into the atmosphere during nuclear testing or accidental emissions. The present invention also relates to a method to detect naturally released radon from environmental samples and to detect radon for long term monitoring.
2. Background of the Invention
A need exists to detect the testing of nuclear weapons in the atmosphere as well as to detect accidental leaks of radioactive materials. Nuclear testing and nuclear power plant accidents release radioactive fission products into the atmosphere. These fission products include isotopes of the noble gases. The United States has entered into a Comprehensive Test Ban Treaty (CTBT).
To monitor compliance with the treaty terms and detect accidental leaks of radioactive material, a means for testing must be available. Currently, one of the best methods for compliance surveillance is by detection of fission products in the atmosphere. Radioactive isotopes of the noble gases provide a good means for detection of nuclear testing. All other fission products are chemically reactive to some extent, and can wash out of the atmosphere by precipitation or combine with other chemicals and precipitate from the atmosphere.
Radioactive isotopes of noble gases disburse in the atmosphere and travel long distances from the site of their release. In the case of underground nuclear testing, most fission products are prevented from getting into the atmosphere. Noble gas fission products emanating from the ground may be the most significant source for radio nuclide monitoring. Among the radioactive noble gases, xenon is the most abundant a few days after detonation. By measuring the atmospheric activities of different xenon radioisotopes (Xe-133, Xe-133m, and Xe-135) as a function of time, a nuclear detonation can be confirmed based on the ratio of these radioxenons exceeding normal ambient levels.
Radioactive krypton and xenon isotopes are primarily gamma and beta emitters. In sufficient concentrations, these isotopes may be readily detected with a range of conventional radiation detectors. For CTBT surveillance purposes, however, the concentrations can be on the order of parts per billion, and do not therefore give sufficient counts to be distinguished from natural background radiation.
The concentration of radon in the environment varies with geography and site conditions. With regard to higher concentrations of radon, many techniques are available for the measurement within an enclosed environment. The U.S. EPA has developed measurement protocols for seven measurement systems. The advantages and disadvantages of those systems are reviewed in the EPA publication, "Radon/Radon Progeny Cumulative Proficiency Report" (EPA-No. 5201/1-86-008), incorporated herein by reference. The seven tested measurement systems include alpha track detectors, charcoal canister gas collectors, continuous radon monitors, continuous working level monitors, grab radon gas sampling, grab working level sampling, and progeny sampling. The most common method is the charcoal canister method. A mechanically dried quantity of activated charcoal having a high surface area to weight ratio is passively exposed to the radon containing atmosphere for a set period of time. The charcoal becomes partially saturated with the gas. The charcoal is then placed in a photo spectrometer and the natural radioactive decay products are measured. In U.S. Pat. No. 4,801,800 the use of a forced air system to concentrate the gas in a shortened period of time is disclosed.
Charcoal has a number of disadvantages when used to collect noble gases. At ambient temperatures, charcoal has a relatively low adsorption coefficient for noble gases which would necessitate very large adsorption volumes to concentrate the parts per billion traces of noble gases in the atmosphere. Charcoal also has a known affinity for water vapor, which decreases its adsorption capacity for noble gases. Airborne contaminants may also build up on the charcoal decreasing its load capacity. Contaminant build up would require equipment designed to regenerate the charcoal while in use. The volume of charcoal required may be lowered by operating at a cryogenic temperature. However, this also adds equipment and any rise in temperature would result in the release of radioactive gases.
The present invention addresses the need to detect low levels of radioactive gas in atmospheric samples. The invention provides a method to concentrate very low level radioactive gases in an organic fluid to efficiently reach detectable levels while overcoming the limitations of prior methods.